My Work At A Glance

I am a researcher and teacher in the
Astronomy Department at the University of Maryland working on a number
of projects. Most recently, I have been
working on Kepler data of a few hundred
galaxies. With the KEGS team we discovered a number
of supernovae: at least four, 5 likely and possibly more.

Other work includes binarity
among stars and a recent discovery of very wide
binaries. In addition, I am working on the absolute magnitude of
red-clump stars, the determination of the distance to the Galactic
center, the detection of solar-system analogs via SIM & Hipparcos
astrometry, and a method of determining one-percent
distances to external galaxies. Furthermore, I'm working on the
stellar content of nearby galaxies in the GALEX data base, as well as
on stars and galaxies in the Kepler field.

Research Interests

EPOXI search for NEOs

Summary plot of a simulation of an EPOXI-follow-up mission
searching for Near Earth Asteroids in Earth-like orbits.
Apparently, there is little appetite for a mission like this in
the Solar System division. This is unfortunate given ease with
which these objects can be detected, their extremely hazardous
nature and the moderate cost of the experiment.

With the EPOXI team, I am currently (fall 2010)
working on a project to define a follow-up mission for NASA's
EPOXI mission.
One of the goals is to search for near-earth asteroids that are
normally lost in the Sun's glare, but which are readily observable
from space. UNFORTUNATELY, THIS MISSION WAS NOT SELECTED.

For an assumed mission lifetime of 10 years,
EPOXI would find roughly 1,000 (60% of all) NEO/Impactors with
diameters larger than 500 m from the Veres 2009 model. This compares
to discovery rate of about 75% for a 4 year survey with the
PanSTARRS-1 system.

On the other hand, Tabachnik & Evans expect of
order 250 objects with diameters exceeding 1,000 m on Earth's
"Tadpole" and "Horseshoe" orbits. These objects are much, much,
much harder to see by ground-based observatories because objects on
these type of orbits can stay away from Earth for 100s to 1000s of
years, even though their orbital elements are very similar to those
of our pale blue dot. Because EPOXI will look at every part of
Earth's orbit in about 7 years, and because these objects get
pretty close to EPOXI, the detection rate for the "Tadshoes" is
essentially ONE HUNDRED PERCENT.

Very Wide Binaries

After many years of on/off
work on new, wide binaries in the solar neighborhood, we recently
completed a pretty stunning piece of work. We discovered new binaries, or former
binaries at heretofore unheard-of separations: up to 10 parsec apart
or 25 degrees. Our method is designed to look for the very widest,
least bound, binary stars, and we have found about 840 probable
candidates that fit that bill. These very wide systems are thought
to have formed during the dissolution phase of low-mass star
clusters. There future (survival as a pair) is
determined by interactions with the tidal field of the galaxy,
close encounters with stars and maybe even dark matter
clumps/MACHOs.

The distribution (Galactic coordinates [centered
on[l=180,b=0]) of our 840 high-probability P(>10%) new wide-binary
candidates. Each primary (red squares) is connected to each of the
possible companions (blue crosses). The concentration towards the
top-middle [at (l,b)~(125,61)] is the UMa group [Monets etal, 2001]:
it contains >30 Hipparcos stars within about 10 degrees.

Astrometry from Space

The former NASA
Space
Interferometry Mission (recommended by two previous Decadal
Committees, but relegated to 11 derogatory lines in a footnote on page
7-9 by the 2010 group) WOULD HAVE REVOLUTIONIZED THE SCIENCE OF
ASTRONOMY. SIM will be able to measure angular sizes to an accuracy of
about 4 micro arcsecond.
To put that in perspective, the continental United States of America
subtends an angle of 4 micro arcsecond at a distance of about 7.6
parsec (~25 light-years =~ 230,000,000,000,000,000 meters).

For example, if SIM-Lite were to look at a Solar
System Analog located 10 parsec away, it would be able to detect the
"reflex motion" that the Earth-twin imparts on that distant sun. As we
know from introductory/high-school physics classes: "action equals
minus re-action." That is to say, The Sun pulls as strongly on the
Earth as vice versa. However, because the Earth is so much lighter
than the Sun (about 333,000 times lighter), the Sun can much more
easily "push the Earth around" (accelerate) than the other way
around. The net effect is that while the Earth moves in an orbit with
size of 150 million km (93 million miles), it makes the Sun move in an
orbit 333,000 times smaller. Doing that calculation (93,000,000 over
333,000), we find that tiny Earth pulls mighty Sun around by 450 km
(280 miles): roughly the distance between Washington DC and New York
NY.

It is indeed truly amazing that SIM-Lite would have detected such
small motions over interstellar distances.

What should have been of particular interest to
cosmologists (7% of AAS members) is the ability of stellar astronomers
(almost 25% of AAS members) to reconstruct quite accurately the
formation history of the Milky Way (a somewhat representative galaxy),
with the data from the Gaia mission. To determine the ages of the
oldest stars in the Milky Way at the percent level requires SIM-like
astrometric accuracy, but this research is now postponed to an
indefinite future (>20 years).

I submitted three White Papers to the Extra-Solar Planet
Task Force. Two of these are on-line.

The first one is on how to use a space mission with a Hipparcos- or
FAME-like scanning pattern to detect 10,000 transiting extra-solar
planets: "We propose a MIDEX-class space mission with the goal to find and
characterize roughly 10,000 transiting planets. When transits occur, a
much more detailed characterization of the planet is possible and so a
large data base of transiting planets will provide planets with a
large range in periods and radii for follow-up studies. Our survey
will be all-sky and focused on stars brighter than V=14.8. Down to
V=12,
LEAVITT will be able to detect Neptune-sized objects. Because of
it's high cadence, LEAVITT is about 100 times more sensitive at
detecting transits than GAIA, while it will find more than 20 times as
many transits as KEPLER. LEAVITT has multi-band photometric capability
implemented via a low-resolution dispersive element which can obtain
0.2% (2 mmag) photometry down to V=14.8. LEAVITT's high multi-band
photometric accuracy reduces the number of false-positives
significantly."

The second one is on detection Solar System Analogs (SOSAs)
by employing SIM-based data in conjunction with HIPPARCOS astrometry:
"The astrometric signature imposed by a planet on its primary
increases substantially towards longer periods (proportional to P^2/3),
so that long-period planets can be more easily detected, in
principle. For example, a one Solar-mass (M_Sun) star would be pulled
by roughly 1 mas by a one Jupiter-mass (M_J) planet with a period of
one-hundred years at a distance of 20 pc. Such position accuracies can
now be obtained with both ground-based and space-based telescopes. The
difficulty was that it often takes many decades before a detectable
position shift will occur. However, by the time the next generation of
astrometric missions such as SIM will be taking data, several decades
will have past since the first astrometric mission, HIPPARCOS. Here we
propose to use a new astrometric method that employs a future, highly
accurate SIM Quick-Look survey and HIPPARCOS data taken twenty years
prior. Using position errors for SIM of 4 muas, this method enables
the detection and characterization of Solar-system analogs with
periods up to 240 (500) years for 1 (10) M_J companions. Because many
tens of thousands nearby stars can be surveyed this way for a modest
expenditure of SIM time and SOSAs may be quite abundant, we expect to
find many hundreds of extra-solar planets with long-period
orbits. Such a data set would nicely complement the short-period
systems found by the radial-velocity method. Brown dwarfs and low-mass
stellar companions can be found and characterized if their periods are
shorter than about 500 years. This data set will provide invaluable
constraints on models of planet formation, as well as a database for
systems where the location of the giant planets allow for the
formation of low-mass planets in the habitable zone."
I just finished a long version of what I thought would be one of my White Papers for
the upcoming Decadal Review. Instead, in collaboration with several
colleagues, we combined into a single White Paper the desire to
promote the extreme utility of both NASA's proposed SIM-Lite
Astrometric Observatory, and ESA's GAIA in the
areas of Galactic archeology
and the final establishment of the Galactic and extra-galactic
distance scales, and the ADS
version.

I am presenting some of my work on astrometry, galaxy-formation
and cosmology via professional colloquia: here is an abstract and a set of detailed slides.

In my talk I will discuss how astrometry, galaxy formation and
cosmology (among others) are intimately linked through the detailed
studies of nearby stars (A) and nearby galaxies (B). Such linking
will be made possible through the highly-accurate astrometric and
photometric data produced by GAIA, the
SIM-Lite
Astrometric Observatory and OBSS (the Origins Billion Star
Survey) .

A) Possibly the most important piece of information that these
missions will nail down stellar ages at the 1% level from the
observed absolute luminosity (distance/parallax) and their masses,
radii and composition: need eclipsing binaries.
-- GAIA can get 1% distances for ~ 190,000 (12,000) [300]
eclipsing binaries in the thin disk (thick disk) [halo] with
accuracies of ~100 Myr per star.
-- SIM-Lite should do the rare special cases such as old Uranium stars.
===> This kind of data will yield the detailed STAR FORMATION
HISTORY and METALLICITY EVOLUTION of the larger Solar
neighborhood (
===> VERY IMPORTANT BENCHMARK for galaxy formation theories

B) At the extra-galactic front, both GAIA and the SIM-Lite Astrometric
Observatory can determine one-percent distances to our closest
neighbors via the method of "Rotational Parallaxes" [Olling,
2007, MNRAS, 378, 1385]. GAIA can do the LMC, SIM is required
for M31 and M33.
I will describe in some detail the RP technique and show why it
is expected to more robust than all other proposed
methods. However, the RP method can sample only the 1st Mpc or
so: other (geometric) methods such as derived by the "Water
Maser Cosmology Project" are essential to bridge the gap between
accurate distances and an accurate H_0.
I will discuss the utility of 1% knowledge of H_0 for cosmology
and dark energy research.

We all though that before SIM flies, we would have a lot of fun
with FAME, which was to be launched in
2004. Unfortunately, FAME was the first victim of NASA's priority
setting mechanism. This is particularly unfortunate because the FAME
data would have provided an excellent data set which, combined with GAIA or SIM,
would have enabled the detection of extra-solar planetary systems much
like our own Solar system.

Other astrometry projects such as DIVA and AMEX did not make
it much past the proposal stage. OBSS (the Origins Billion Star
Survey) was one of nine selected proposals from a field of 26
submissions for detailed feasibility studies for the next generation
of space probes in its Astronomical Search for Origins Program in
2004. Contrary to other astrometry missions, OBSS is designed to
combine wide-field imager with a rapid re-pointing technologies to
obtain a high-cadence observatory. OBSS will work much like a
ground-based astrometry program, where the absolute astrometry will be
obtained by direct linkage to an extra-galactic reference
frame. Because OBSS is a pointed instrument, it can in principle
obtain very long integration times (and hence very good accuracy) for
selected sources. GAIA is
expected to yield accurate
astrometry, photometry and radial velocities for one billion
stars, and will revolutionize our understanding of
astrophysics: from brown dwarfs to cosmology. Of course,
astrometry was re-invented by Hipparcos,
the first satellite dedicated to astrometry.

My Contributions to (Space) Astrometry @ USNO

During this period, I worked on many
aspects of astrometric and photometric surveys of the Galaxy and the
Local Group. Most (25) of the resulting technical memoranda are listed
below. None of these memos have been published.

Black Hole Results from STIS

The Space Telescope Imaging Spectrograph (STIS) has obtained
0.1-0.2'' resolution spectra from the nuclei of about 15 nearby
galaxies in a search for supermassive black holes. This talk concentrates on the data
reduction process and the difficulties which have to be overcome to
obtain reliable kinematic measurements from STIS observations. In
particular, the under-sampling of the spatial part of the point-spread
function and the presence of a large numbers of cosmic rays in the
images complicate the analysis significantly. Our analysis is based
on standard STIS-pipeline software (the IDL version of the CALSTIS
package). We pay particular attention to the effects of regridding
under-sampled images containing galaxies (with power-law cores) as
well the point-like artifacts that typically contain ~90% of the total
counts (cosmic-ray hits).

We also present the nuclear kinematics for several galaxies for
which Rutgers astronomers are lead investigators, including M32, M87,
NGC 2842. In M32 we reproduce the van der Marel et al. (1998) FOS
results but with approximately 7 times higher velocity resolution,
and 2 times higher spatial resolution. In NGC2841 we obtain a clear
signature in the stellar motions of a black hole with mass of several
tens of million solar masses, the first black hole detection in this
galaxy. In M87 we measure the stellar velocity dispersion at a radius
of ~0.3'', a factor of ~2 higher spatial resolution than existing,
ground-based data. In each of these galaxies, the STIS stellar data
extend well within R_g, the radius of gravitational influence of the
black hole, making the interpretation of the black hole mass
essentially independent of the stellar anisotropy.

Flattened Dark Matter Halos

Currently I am trying to determine the shape of dark matter
halos surrounding spiral galaxies. This is done by constructing
(almost) self-consistent mass models of spiral galaxies which have the
shape of the halo as a free parameter. Galaxies with flat halos have
thinner gas layers than galaxies with a round halo, for a given
rotation curve and gaseous velocity dispersion ( 1995,
AJ, 110, 591-612 ). Observationally, sensitive high resolution HI
spectral line observations yield the rotation curve, the thickness and
velocity dispersion of the gas layer ( 1996,
AJ, 112,457-480 ). Thus, these observations allow for the
determination of the shape of dark matter halos of spiral galaxies
(with extended HI envelopes). The first results, for the edge-on Scd
galaxy NGC 4244 ( 1996,
AJ, 112, 481-490 ), indicate that dark halos might be highly
flattened (shortest-to-longest axis ratio ~0.2, or E5-E9 shape, for
NGC 4244). Details about mass modeling of spiral galaxies and
determining the thickness of the HI layer can be found in my thesis
which is available via ftp.

There are also dynamical effects of a flaring gas layer as illustrated
by the figures on the left. The top panel shows the geometry of a
line-of-sight that passes through an arbitrary point in the
galaxy. Before the line-of-sight intersects the midplane, it passes
through gas closer to the major axis (smaller angle theta) but at
greater z-heights above the plane. Because this gas is closer to the
major axis, it has a larger value of the "cos(theta)" projection
factor: that is to say, larger apparent radial velocity. Gas beyond
the midplane arises from larger theta-angles and has hence smaller
"cos(theta)" values. Thus, along a given line of sight, there is a
radial velocity gradient induced by the flaring, with larger
velocities closer to the major axis. In the limiting but unrealistic
case that the HI densities would be the same all along the line of
sight, there would not be a net velocity bias. However there are
likely to be several gradients present: 1) vertical density gradient,
2) radial surface-density gradient, 3) radial rotation-curve gradient,
and 4) radial velocity dispersion gradient (as well as possible
vertical gradients of #3 and #4). However, the strongest gradient is
due to the finite thickness of the disk (#1), and as long as this
gradient dominates, the vertical structure of the disk will set the
effects on the velocity field. Analytical calculations show that: 1)
in all cases the peak intensity is shifted towards higher velocity
[i.e., smaller "cos(theta)" factors], 2) thicker gas layers cause a
larger shift, 3) steep radial density profiles increase the effects,
and 4) the effect decreases towards smaller inclinations.

To illustrate this effect, we computed a model spectral-line data cube
and a corresponding velocity field with parameters similar to the
fitted parameters for NGC 3198 (Begeman 1987). However, we increased
the inclination to 80 degrees and we used a completely flat rotation
curve. The computed velocity field (top-right panel of the
accompanying figure) shows an outward curving of the iso-velocity
contours, which is indicative of a rising rotation curve. Since our
input was a flat rotation curve, the difference can be fully
attributed to the effects of the flaring of the gas layer. We also
performed fits (employing ROTCUR) to the resulting velocity field and
we display the results in the bottom panel of the accompanying
figure. Here we see that the inferred inclination is smaller than the
input value, and that the flat input rotation curve is transformed
into an apparently gently rising rotation curve ( Olling & van Gorkom, 1993).
Click here for pfdf file

Since these effects are largest in the outermost regions of galaxies
(where the column-density gradients are largest) and for low-mass
galaxies (with thick gas layers), the current interpretation of
rotation curves as derived from the velocity fields in terms of the
amount and shape of dark matter will be affected to a currently
unknown degree.

The Milky Way

Size, Mass and Shape of the Milky Way Mike Merrifield and I
are using a similar technique for the Milky Way Galaxy. We combine
this method with constraints arising from the total mass within 1.1
kpc from the plane of the Galaxy. We find that the dark halo of the
Milky Way is rather round, with shortest-to-longest axis ratio (c/a)
>~0.5 if R_0 >~7 kpc. The inferred dark matter halo flattening
depends strongly upon the distance to the Galactic center and local
rotation speed. The measurements of the thickness of the gas layer in
the outer Galaxy exclude the IAU recommended values of R_0 = 8.5 km/s
and Theta_0 = 220 km/s (assuming the IAU recommended value of 26.4
km/s/kpc for Theta_0/R_0=A-B). Using the local stellar column density
as a constraint, we find that the distance to the Galactic center is
smaller than ~7.3 kpc. Similarly, assuming that the Milky Way's Dark
Matter Halo is oblate we deduce that Theta_0 <= 188 - 5.6*(R_0-7) +
4*(R_0-7)^2.(Olling & Merrifield, 2000, MNRAS,
311, 361). (postscript, html (not yet available,
sorry)).

Luminous
and Dark Matter in the Milky Way

We have extended the analysis
above to include the effects of the (unknown) temperature of the
interstellar medium. We find that the temperature gradient of the ISM
in the Galaxy has to be small, so small that is can not affect the
above conclusions significantly. The Milky Way's dark matter halo can
be significantly flattened only if our distance to the Galactic center
is smaller than ~6.8 kpc. So the dark matter in the Milky Way is
probably not in the form of cold molecular hydrogen or decaying
massive neutrinos as these forms of dark matter require very flattened
distributions (c/a < 0.2). If we assume the IAU-recommended values
for the Galactic constants, it is NOT possible to build a
self-consistent Milky Way mass model, unless the plane of the Galactic
dark matter halo is perpendicular to the plane of the Galactic
disk. We have recently submitted
these results to MNRAS. (postscript, html (not yet available,
sorry))

Popular descriptions of this work can be found in various places:
the 1997 NAM press
release, the report in El Pais, or in Science
Now). We are also re-examining the Oort constants, how they
are determined and what we can learn from them. Completely independent
from the halo flattening results, we find strong evidence that the
distance to the Galactic center and the rotation speed at the Solar
circle are smaller than commonly thought: R_0 = 7.1 +/- 0.4 kpc, and
Theta_0 = 184 +/- 8 km/s. (1998,
MNRAS 297, 943 , postscript , html (not yet available,
sorry))

My
Literature Database

I try to keep up with the literature on a number of topics. These
days, most paper I read are stored in an “ADS
Personal Library,” with over 278 topics of interest to me,
and over 7,533 papers that I "read" (i.e., at least the abstract). I
encourage you to create such libraries yourself.

Last Updated: May 2015

General
Interests

Hiking, astronomy, Belgian and US micro-breweries, speed
skating.To understand why Dutch people like ice skating, check out
Siebren's pictures of Nederland
onderijs. Or a collage of pictures from the 1997
Elfstedentocht.

IDL Speed Tests

My IDL speed test routine
is taken from the standard IDL library, and modified to be slightly
more general. Details of the test can be found here.